Modern electrical therapeutic and diagnostic devices for the heart, such as pacemakers, cardioverters and defibrillators for example, require a reliable electrical connection between the device and a pre-selected region of the heart. Typically an electrical "lead" is used for the desired electrical connection.
One type of commonly used implantable lead is an endocardial lead. Endocardial leads are attached at their proximal end to an implantable pulse generator and at their distal end to the endocardium of a cardiac chamber. Such leads normally take the form of a long, generally straight, flexible, insulated conductor having one end electrically connected to the pulse generator and the other end electrically connected to the endocardium through an electrode. Among the many advantages of an endocardial lead is that it may be positioned into the heart by sliding the lead through a vein until the electrode is properly positioned, rather than physically exposing the heart itself.
The specific design of the endocardial lead used has often varied depending upon the region of the heart to which it is to be connected, in particular whether it is for a ventricular application or an atrial application.
Ventricular endocardial leads are often readily flexible and have tines or fins at their distal end. These tines are provided to engage the trabeculation within the ventricle so as to reliably fix, or at least position, the electrode in the desired location. Unlike the ventricles, the atrial walls are relatively smooth. Because the atrial walls are smooth it has been difficult to retain the electrode in a fixed position with respect to the wall of the atrium. One approach commonly used has been to form the distal end of an atrial lead in a J-shaped configuration. Such a configuration causes the distal end to curve upwardly once the lead is within the atrium so as to provide reliable contact between the electrode and the heart tissue.
In dual chamber pacing, however, it is necessary to establish an electrical connection with both chambers of the heart. Typically this now involves the placement of two leads, a ventricular lead as well as an atrial lead, within the patient's heart. Usually the ventricular lead is placed first, i.e. it is passed through a blood vessel and into the ventricular cavity. When the ventricular pacing lead has been stabilized within the heart, the second lead, or atrial lead, is passed through the blood vessel and is moved into a selected position within the atrial cavity.
The placement of two separate pacing leads into two separate chambers of the heart, however, is a relatively complicated procedure. First as the second lead is being inserted, it is possible to strike the first lead with the second lead thereby dislodging the first lead from its desired position. In addition, the presence of two leads may cause a significant decrease in blood flow through the blood vessel, especially in patients having relatively small diameter vessels. Finally, although transvenous placement of a lead is relatively not traumatic, it would nonetheless be beneficial to simplify and shorten the implant procedure as much as possible. Reducing the number of leads implanted from two to one would be of significant benefit.
Because of the difficulties encountered by placing two leads there has been a considerable number of past attempts to design a single lead which provides an electrical connection to both chambers of the heart, often referred to as a "single pass lead." An early attempt at a single pass lead was taught by Bures in U.S. Pat. No. 3,865,118. Because the configuration taught by Bures requires the ventricular lead to be coaxially mounted within the outer sheath, minimal control could be exercised over placement of the atrial electrodes. To compensate for this lack of control, Bures taught the use of opposing (i.e., spaced by 180 degrees) spring loaded electrodes. Such a placement technique is susceptible to dislodgement, however. It is also electrically inefficient because of the relatively large surface area of the electrode and the difficulty in controlling the amount of that surface area actually in contact with the atrial wall. Furthermore, using the outer catheter to control flexure of the atrial electrodes lead to sealing problems.
Lajos in U.S. Pat. No. 4,057,067 attempted to solve many of the control problems found with the lead taught by Bures by using a "J" shaped atrial lead with stylet control. Because the atrial and ventricular leads, however, were spaced a fixed distance, the lead taught by Lajos did not accommodate various sized hearts. A further problem with the Lajos lead was the establishment of an effective seal of the hole at the distal end of the atrial electrode. During insertion, this hole is blocked by the stylet. Removal of the stylet, however, permitted seepage of blood into the lead.
A third single pass lead configuration was taught by Sabel in U.S. Pat. No. 3,949,757. Sabel used the "J" shaped atrial electrode placement as taught by Lajos but slid the atrial catheter within the outer sheath of the ventricular catheter. This solved one problem of Lajos by not requiring an aperture in the distal end of the atrial electrode for stylet straightening of the "J" shape. It did not completely solve the problem of differing heart sizes, however. The distance between the distal end of the atrial catheter and the distal end of the outer sheath was essentially fixed by practical factors even though the atrial catheter was slidably mounted within outer sheath because sliding of the atrial catheter also changed the shape of the "J". The atrial electrode may be lowered in the atrium by moving the atrial catheter either proximal or distal relative to the outer sheath. However, the atrial electrode may not be raised within the atrium. That distance is effectively established by the prior implantation of the ventricular electrode. Providing a larger distance between the ventricular electrode and the distal end of outer sheath would tend to distort the "J" shape of the atrial catheter.
Another proposed configuration for a single pass lead was disclosed by Gold in U.S. Pat. No. 4,444,195 which disclosed a flexible catheter having a series of ring electrodes selectively utilized for pacing and sensing in both chambers of the heart. As discussed above, one significant problem with this configuration was the reliable, consistent and acceptable placement of the atrial electrodes.
A still further attempt to configure a single pass lead was disclosed by Harris in U.S. Pat. No. 4,627,439 which featured a single pass lead having a prebent atrial section. In particular the atrial section had a bend with the electrodes positioned on the bend. The bend, it was taught would assist in properly maintaining the position of the atrial electrodes. The Harris design, however, failed to provide an acceptable single pass lead. In particular the configuration of the prebent section having electrodes on the bend failed to provide acceptable chronic electrode position.